Fuel cells are the modern electrochemical devices that convert the chemical energy of a fuel into electric energy and heat energy with high efficiency [S. C. Singhal, Solid State Ionics 135, 305-313 (2000); A. Weber, E. Ivers-Tiffée, J. Power Sources 127, 273-283 (2004); E. Lust, P. Möller, I. Kivi, G. Nurk, S. Kallip, P. Nigu, K. Lust, J. Electrochem. Soc. 152 (2005) A 2306; O. Yamamoto, Electrochim. Acta 45 (2000) 2423]. The basic structure of a fuel cell consists of the porous cathode (oxygen reduction process) and porous anode (oxidation—fuel burning process) and a compact electrolyte layer between cathode and anode. In a typical fuel cell, fuels in a gaseous phase, typically hydrogen, methane etc. are continuously fed to the anode electrode compartment and an oxidant typically oxygen from air (or pure oxygen) is continuously fed to the cathode electrode compartment [S. C. Singhal, Solid State Ionics 135, 305-313 (2000); A. Weber, E. Ivers-Tiffée, J. Power Sources 127, 273-283 (2004); E. Lust, P. Möller, I. Kivi, G. Nurk, S. Kallip, P. Nigu, K. Lust, J. Electrochem. Soc. 152 (2005) A 2306]. The electrochemical oxidation and reduction reactions take place at or inside the porous structure of electrodes to produce an electric current and residual heat because the exothermic fuel oxidation reaction takes place as well as clean water vapour as a final chemical product forms.
In a solid oxide fuel cell the electrolyte is a nonporous compact mixed metal oxide, traditionally Y2O3-stabilized ZrO2 (YSZ) for so-called high-temperature solid oxide fuel cells (working temperature T>1073 K) and Sm2O3-stabilized CeO2 (CSO) or Gd2O3-stabilized CeO2 (CGO), for the so-called intermediate temperature (773<T<973 K) solid oxide fuel cells. For high-temperature solid oxide fuel cells the cathode is typically Sr-doped LaMnO3, but for intermediate-temperature solid oxide fuel cell the cathode is typically Sr-doped LaCoFeO3 or Sr-doped LaCoO3, where the mixed conduction process of the oxygen ion occurs. The anode electrode is the metal (Ni or Cu)/YSZ cermet for the high-temperature solid oxide fuel cell (973<T<1273 K) and Ni/CSO cermet for the intermediate temperature solid oxide fuel cell (773<T<973 K) [S. C. Singhal, Solid State Ionics 135, 305-313 (2000); A. Weber, E. Ivers-Tiffée, J. Power Sources 127, 273-283 (2004); E. Lust, P. Möller, I. Kivi, G. Nurk, S. Kallip, P. Nigu, K. Lust, J. Electrochem. Soc. 152 (2005) A 2306; O. Yamamoto, Electrochim. Acta 45 (2000) 2423].
The most commonly used solid oxide fuel cell cathode material is mixed-conducting La1-xSrxMnO3 (Sr-doped LaMnO3) oxide, where x denotes the molar ratio of strontium added into LaMnO3. However, the rate of electroreduction of oxygen from air is a very slow process and one possibility to increase the catalytic activity is to use the electrochemically more active La1-xSrxCoO3-δ or La1-xSrxCo1-yFeyO3-δ cathodes, where y is the molar ratio of Fe3+ ions added into LaSrCoO3 [S. C. Singhal, Solid State Ionics 135, 305-313 (2000); A. Weber, E. Ivers-Tiffée, J. Power Sources 127, 273-283 (2004); E. Lust, P. Möller, I. Kivi, G. Nurk, S. Kallip, P. Nigu, K. Lust, J. Electrochem. Soc. 152 (2005) A 2306; O. Yamamoto, Electrochim. Acta 45 (2000) 2423; V. Dusastre, A. Kilner, Solid State Ionics 126, 163-174 (1999); A. Esquirol, N. P. Brandon, J. A. Kilner, M. Mogensen, J. Electrochem. Soc. 151, A1847-A1855 (2004)]. Another possibility is to increase the reaction volume (reaction area) through nano(micro)mesoporous structure of the cathode electrode layer. The nanopores are pores with width lower than two nanometers, named according to IUPAC classification as micropores.